1. Field of the Invention
The present invention relates to a thin film transistor (hereinafter, abbreviated as TFT) of a bottom gate type which is fabricated by utilizing a semiconductor thin film.
2. Description of Related Art
Currently, a liquid crystal display device (hereinafter, abbreviated as LCD) is utilized as the display for notebook personal computers or portable information terminals. In the past, a passive type LCD was mainly used. However, with requests for highly fine images becoming more enhanced, an active matrix type LCD has become the main stream.
An active matrix type LCD is provided with a plurality of TFTs arranged in a matrix in a display unit thereof. Voltage control is carried out at each pixel with a TFT as a switching element by which the desired image is realized. An inverted stagger type TFT using amorphous silicon for an active layer has been frequently utilized.
For such an inverted stagger type TFT using amorphous silicon, the following advantages are realized: simple fabrication steps, high throughput, excellent yield and the like. However, the carrier mobility of amorphous silicon is small, and accordingly, requested electric properties are provided by enlarging the size of the individual TFT.
However, currently, the size of TFT is required to be in order to realize a highly fine image display. Situations in which electric properties capable of meeting the request are very difficult to provide in the case of amorphous silicon.
Hence, in recent years, an inverted stagger type TFT using polysilicon for an active layer has attracted attention, and keen competition for development thereof has been carried out. In recent times, while active matrix type LCDs each constituted by an inverted stagger type TFT using polysilicon are found in the market, the product is regarded to still be in a developing stage.